The present invention relates to arrangements whereby the hazards of so-called "boiling liquid expanding vapor explosion" (which is referred to herein by the initials BLEVE) may be reduced or avoided. More particularly, the present invention is concerned with measures for the protection of closed storage vessels for the storage of liquefied gases at ambient temperature. Examples include large scale storage vessels for the storage of non-flammable liquefied gases such as liquefied inert gases, e.g. the FREONS and other inert volatile aerosol propellant liquids and refrigerants, liquefied petroleum gas, liquefied ammonia and liquefied propane. The invention is, however, in general applicable to the protection of all closed storage vessels for liquefied gases having their critical temperature above normal ambient temperature whether flammable or non-flammable where the wall structure of the vessel is capable of withstanding a significant internal pressure. Such vessels will typically have pressure-resistant walls adapted to withstand internal pressures of at least 200 p.s.i.g., more typically at least 400 p.s.i.g. All such storage vessels are liable to the said explosive failure of enormously destructive capability, termed a BLEVE, on accidental exposure to a fire hazard or other source of heating impinging on the exterior of the vessel.
As a result of the application of heat, a pressure is built up within the vessel. If the flame or other heating is initially incident on an area of the vessel below the liquid level, the heat will initially be dissipated through the liquefied gas boiling, so that the internal pressure increases. If the vessel has a pressure-relief valve, once a certain preset pressure limit is reached, vapor will be released through actuation of the pressure-relief valve. However, since the conventional pressure relief valves vent through a restricted opening, the rate of heat flow and of boiling-off vapor may rapidly overwhelm the capacities of the pressure-relief valve so that an internal pressure greatly in excess of the preset pressure limit may be achieved. Once sufficient liquid has evaporated, and the liquid level has dropped, that region of the wall of the vessel that is now above the liquid level will become rapidly hotter, and the tensile strength of the metal or other material constituting the wall of the vessel in this region will be locally weakened to a point where the vessel wall ruptures explosively under the internal pressure that has built up within the vessel.
A serious problem associated with the BLEVE is that following the initial accident that results in an outbreak of fire, there is a period of unpredictable duration, before the heating is sufficient to weaken the metal skin to the point where the pressure vessel ruptures.
Up to the present invention, there has been no satisfactory way of preventing or adequately deferring the occurrence of a BLEVE.
Proposals have been made to coat the exterior of railroad tank cars and the like with insulative thermal shields in an attempt to resolve explosion problems. The difficulty with the exterior coatings is that they are likely to be broken off or destroyed as a result of impact while leaving the metal wall of the tank intact. Thus, under the conditions in which the tank cars are most likely to be exposed to a heating hazard, i.e., through collision or derailment, the tank wall is liable to be left intact and exposed to a heating hazard, e.g., to burning gases escaping from an adjacent damaged tank, thus exposing the intact tank to risk of explosive rupture.
It is known to employ explosion-suppressive inserts of reticulated plastic foam or of expanded aluminum foil in fuel tanks which in service contain explosively ignitable mixtures of fuel vapor and air. It is known that these inserts reduce the transient rise in pressure which occurs within the container when the combustible mixture is ignited, so that the resulting rise in pressure, termed the "overpressure", is insufficient to burst or yield the wall of the container. An example of a reticulated plastic foam inert material is described in Allen U.S. Pat. No. 3,561,639, and an expanded aluminum foil insert is described in Szego U.S. Pat. No. 3,356,256.
The above explosion-suppressive inserts have been used in tanks which contain an explosively combustible fuel vapor/air mixture. They have not been employed in pressurised liquefied petroleum gas storage vessels or other storage vessels containing ambient temperature pressurised liquids which are stable when heated.